Microchip charge patterning

ABSTRACT

A method of forming a charge pattern on a microchip includes depositing a material on the surface of the microchip, and using an external device to develop charge in the material.

BACKGROUND

Systems exist that assemble microchips automatically based upon somesort of encoding such as electrostatic or magnetic coding. The systemuses the encoding to identify and position the microchips as part of theassembly process.

Examples of these systems can be found in U.S. Pat. No. 7,861,405, andits related cases, US Patent Publications 20100192365, 20100186221, and20100186222, owned by the assignee of this application. Another exampleis shown in U.S. Pat. No. 7,332,361, that teaches charge-encodedelements having basic or acidic surfaces capable of carrying charge innon-polar liquids with charge control agents. Examples includetwo-particle electrophoretic ink, liquid toner, inorganic oxides andpolymers. Standard photolithography or ink-jet technology can be used topattern these materials to form charge-encoded elements. All of thesereferences are incorporated by reference here. The system uses a chargeor magnetic polarity on the chip to sort and position the microchips.

These references make general mention of possible methods of how chipshave the charge or polarity, but give no details. For example, issuescan arise with the deposition of the charge or polarity. Currently, nocurrent technique exists to deposit a charge pattern on isolatedmicrochips. One approach has charged one end of a nanowire, another hascharged up one side of a symmetric microchip, but no work appears to bedone on isolated microchips or charge patterning.

The fabrication process should be compatible with existing devices onthe chips. In addition, the process should not increase themanufacturing costs, but still allow the patterning on a scalable levelbetween larger areas and small features. In order to provide a flexiblesystem, the system should allow for varying magnitudes of chargedensity, and ideally but not necessarily, both positive and negativepolarity, for multiple levels of chip control and identification. Thecharge patterns should have good stability to allow long shelf life ofthe patterned devices.

Another issue with patterning may arise with the chips being attractedto each other. They may gather together or conglomerate based upon theirrespective charges. The density of the chips needs to remain low enoughthe chips do not exchange charge by contact with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 show an embodiment of an overall method of forming chargepatterns on microchips.

FIG. 2 shows an embodiment of a method of forming charge patterns on amicrochip.

FIG. 3 shows an embodiment of a method of printing charge patterns on amicrochip.

FIGS. 4-7 show an embodiment of a method of photolithographicallyforming charge patterns on a microchip.

FIGS. 8-10 show an embodiment of a method of forming a charge patternusing a scorotron.

FIGS. 11-13 show an embodiment of a method of forming a bipolar chargetemplate.

FIG. 12-16 show an alternative embodiment of a method of forming abipolar charge template.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows an overall embodiment of a method of forming chargepatterns on a microchip. At 11, the microchips are fabricated on asubstrate, such as silicon, gallium arsenide, complementary metal oxidesemiconductor (CMOS) etc. The process then deposits materials on themicrochips to enable charging at 13. In one approach, shown in moredetail in FIGS. 2-7, the microchips are then submersed in a fluid thatcauses charge to develop on the materials deposited. In anotherapproach, shown in more detail in FIGS. 8-16, the materials on themicrochips are charged using an external device. The end result ismicrochips with charge patterns on their surfaces in 19.

FIG. 2 shows an embodiment of a method of forming charge patterns on amicrochip. The process will more than likely begin at the wafer level at10, after manufacture of the individual microchips on the wafer. Thisallows the process of charge patterning to occur in a more mass-producedmanner, using techniques compatible with chip fabrication processes.However, no limitation is intended by this discussion and none should beimplied. The individual chips can be patterned using the same techniquesafter singulation from the wafer.

Using manufacturing techniques compatible with chip processing, themanufactured microchip 12 has a shield 14 to protect the circuitry onthe chip. The shield has an insulator 16. The insulator allows charge ormaterial patterns deposited on the surface to avoid interaction with thecircuitry on the microchip. The pattern ABBAB may represent differentcharges, such as +−−+−, or a pattern of materials into which charge willbe developed. Through the techniques discussed in more detail further,the charge pattern may consist of alternating regions of differingcharge magnitude and/or polarity. The embodiment in this portion of thediscussion has the patterns being charge.

Once singulated, the microchips can be placed into a fluid to create an‘ink’ bottle 22 or other dispenser. The fluid may contain otheradditives such as a charge control agent. The charge control agent mayeither allow the charge to remain stable on the surface of the chips 12,or may cause the charge to develop on the surface of the wafer. Thecharge generation may take the form of depositing a material, anddeveloping a charge in the material thorough the interaction of thecharge patterning material with the fluid component of the ink.

One approach uses deposition of a thin-film of the charge patterningmaterial using solution processing techniques such as spin-coating,printing, dip-coating or self-assembly, or vapor deposition techniquessuch as plasma enhanced chemical vapor deposition (PECVD) or atomiclayer deposition (ALD), followed by immersion in solution afterdeposition of the film. Examples of materials that could be deposited bysolution or vapor techniques include polymers such as polyethylene,polystyrene, polymethylmethacrylate, and parylene and polyvinylalcohol,as well as cationic and anionic polyelectrolytes such aspolystyrenesulfonic acid, polyallylamine, polyacrylic acid, andpoly(diallyldimethylammonium chloride). Other examples includeorganometallic salts such as zinc stearate or aluminum stearate as wellas oxides, such as silica, alumina or titania. In addition materialswhich foam self-assembled monolayers such as octadecyltrichlorosilane,phenethyltrichlorosilane, hexamethyldisilazane, allyltrimethoxysilane orperfluorooctyltrichlorosilane may also be used.

The charge patterning material may not be a pure material but rather ablend of one or more materials, possibly to enhance charge generatingproperties, improve processing performance, or impart new functionalityto the film such as making it light sensitive. For materials depositedfrom solution, they would likely be first dissolved in a solvent such astoluene, hexane, water, isopropanol, or tetrahydrofuran to enablesolution processing. An example process may involve spin coating thematerial onto the surface of the microchip, patterning the materialeither using light directly if a light-sensitive formulation of thecharge-patterning material is used, or in a separate step throughphotolithographic/etching cycles, and immersing the chip into solutionwhere free charges are formed through interaction between the fluid andthe material.

In FIG. 4, a first polymer is spun onto the insulator 16 andphotolithographically defined in FIG. 5, exposing a portion of theinsulator 16. A second polymer 42 is then spun on or otherwise depositedon the first polymer and the exposed insulator in FIG. 6. The secondpolymer is then patterned to define a region separate from the firstregion, typically with a region of the insulator. The two differentpolymers then retain different charges, or magnitudes of charge to forma charge pattern. In addition to the use of two solution processedpolymers, other embodiments may use a combination of a solutionprocessed material to generate one polarity/magnitude of charge and avapor deposited material to generate another polarity/magnitude ofcharge, other embodiments could use two different vapor depositedmaterials.

If immersed in a polar liquid, such as water, most surfaces becomecharged, as some of the surface molecules dissociate. Charge tends to bemore negative or positive depending on the chemical composition of thesurface. Typical functional groups that develop a negative surfacechange include sulfonic acids, phosphonic acids, and carboxylic acidsamongst others. Typical functional groups that develop positive surfacecharges include amines and imidazoles, amongst others. The range ofCoulomb interaction of the charged microchips is controlled by the ionicstrength of the solution, but is typically rather short, in the range of1 to 10 micrometers due to the high ion content.

In this embodiment the use of non-polar fluids is desired however, sincesuch Coulomb interactions between charged entities are felt over muchlarger length scales, in the range of 10-100 micrometers. Examples ofnon-polar fluids include isoparaffinic liquids such as the isopar seriesof fluids, and other hydrocarbon liquids such as dodecane. In non-polarliquids charge control agents, typically amphiphilic surfactantmaterials (both ionic and non-ionic), such as phosphatidylcholine(lecithin), sorbitan monooeleate (span-80), aluminium-di-tert-butylsalicylate (ALOHOS), polyisobutylene succinimide (OLOA), or sodiumdioctylsulfosuccinate (AOT) amongst others may need to be added to theliquid to assist in developing charge.

If the surrounding liquid is air, charging of the surface can be donethrough tribo-actions. For example, in Xerography, the toner particlesare charged by contacting specifically designed developer particles. Thecharge patterning material has to be selected such that it charges inspecific ways when contacted with the developer material. The density ofthe chips should stay low enough to control charge exchange betweenmicrochips accidentally hitting each other.

In another approach, shown in FIG. 3, an ink-jet head 30 can deposit thecharge-patterning material. Candidate material may include waxes,polymers, such as polyelectrolytes, or blends of materials amongstothers. After deposition of the charge patterning material, as inprevious descriptions the chips would be singulated and immersed in afluid where free charges would be generated.

In another approach, a self-assembled monolayer can be used along withanother material, such as an oxide, to create charged regions on thechip. This could be accomplished, for example, by first depositing theoxide, then depositing the self-assembled monolayer having the desiredfunctionality over the oxide. Using photolithography and etching thisself-assembled monolayer could then be patterned revealing the oxideunderneath. In another approach, the oxide could be deposited followedbe patterning of a sacrificial blocking material (such as photoresist),deposition of the self-assembled monolayer could then take place,followed by removal of the sacrificial material, again revealing theunderlying oxide surface, creating an oxide, self-assembled monolayerpattern. This second approach has the benefit in that the self-assembledmonolayer deposition and blocking material removal steps can be carriedout either at the wafer scale, or at the chiplet scale after wafersingulation.

FIGS. 4-7 show processes of an embodiment in which photolithographicpatterned polymers, inorganic materials or a mixture of the two areused. These materials can either be tribo-charged or chemically chargedusing a charge control agent, in a dielectric fluid to carry differentpolarities of charge. Photolithographic defined charging of a dielectricwith a corona device is another approach, using dry film resist.Commercially available dry film resists express substantial conductivitydifference between exposed ultraviolet and unexposed regions. FIGS. 4-7show a process for photolithographic materials.

In a photoconductor coating embodiment, one may use inorganicphotoconductor materials such as amorphous silicon and selenium, orsingle layer or multilayer organic photoconductors. If such aphotoconductor coating is used, it receives charge from a corotron orscorotron to give the desired charge signs and patterns. As the termsare used here, a corotron is a corona charging device typically used inelectrophotography/Xerography that has a wire connected to a highvoltage power source. In a typical application, the wire generates anelectric field that charges up a photoreceptor in preparation forreceiving charged toner particles. A scorotron is a screen corotron.

Another embodiment uses photolithographic define charging of adielectric with a corona device. Commercially available dry film resistexpresses substantially conductivity difference between ultravioletexposed and unexposed regions. FIGS. 8-10 show one embodiment of such aprocess. In FIG. 8, a dry film resist is laminated onto the insulatorlayer 16 over the microchip or microchip wafer 12. In FIG. 9,ultraviolet light exposes a portion 52 of the dry resist, creating highand low conductivity regions. In FIG. 10, the wafer or microchip 12 isgrounded by connection 54 and charged with a scorotron or corotron 56,to form charge patterns based upon the previously exposed regions 50 and52 of the dry film resist. This forms a single charge template, wherethe surface has one polarity of charge. One may verify the chargepatterns by spreading beads of known charge and imaging with a camera,or measuring the field directly in air with a non-contact electrostaticvolt meter.

FIGS. 11-13 show an embodiment of a method of forming a dual chargetemplate. Ultraviolet light exposes a portion of the dry film resist 50to form a low conductivity region 52. Using a stencil mask 62 on top ofthe dry film resist allows exposure of only a portion of the lowconductivity region formed by the UV exposure. The scorotron 64 appliescharge of a first polarity to a portion of the low conductivity regionnot protected by the mask 62. The process shifts the mask 62 to coverthe charged portion of the low conductivity region. The scorotron thencharges the uncharged portion of the low conductivity region with alower dose of the opposite sign ions to create the other sign chargepattern. In order to differentiate between the two scorotron processes,the opposite sign charge pass of the scorotron is referred to as 66.

In an alternative embodiment, a bipolar or a dual charge template canresult from a dual exposure, shown in FIGS. 14-16. FIG. 14 shows a firstexposure forming a first low conductivity region 52 on the dry filmresist 50. This first region then receives a first polarity charge froma scorotron 62. The dry film resist then receives a second exposure ofUV light to form a second low conductivity region 70. This second regionthen receives a charge of the opposite polarity from the scorotron 64.

In this manner, charge patterns can be generated on the surface ofmicrochips, either before or after singulation from a wafer. Thetechniques used are compatible with semiconductor fabrication processes,either using scorotron or corotron charging, tribo-charging, or byapplication of charged materials. Free charges may then be generateddirectly or through the interaction of these materials with thesurrounding fluid in the chip “ink.” The charge patterns containinformation usable in automated systems to sort, organize and arrangemicrochips into larger circuits.

It will be appreciated that several of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations, or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

What is claimed is:
 1. A method of forming a charge pattern on amicrochip residing on a wafer, comprising: depositing a material on thesurface of the microchip; and using an external device to develop chargein the material; and singulating the wafer into microchips aftercharging.
 2. The method of claim 1, wherein using an external device todevelop the charge comprises charging the material with one of either acorotron or a scorotron.
 3. The method of claim 1, wherein depositingthe material comprises depositing a photoconductor.
 4. The method ofclaim 1, wherein depositing the material comprises laminating a dry filmphotoresist onto a wafer and exposing the dry film photoresist toultraviolet light, and developing the charge comprises charging theexposed photoresist with a scorotron.
 5. The method of claim 4, whereinexposing the dry film comprises applying a stencil mask to the dry filmresist to expose a portion of a low conductivity region and applying afirst charge with a scorotron.
 6. The method of claim 4, furthercomprising re-exposing the dry film resist with a second charge with ascorotron.
 7. The method of claim 1, wherein the depositing of thematerial comprises depositing one of a polymer, an inorganic material ora mixture of an inorganic and a polymer.
 8. The method of claim 7,wherein developing the charge comprises tribo-charging.